The present invention relates to an image reader in which the sensitivity fluctuations of a solid-state image pickup device which may be caused in reading signals therewith are decreased.
A solid-state image pickup device including, for instance, a charge-coupled device (hereinafter referred to as "a CCD", when applicable) is, in general, employed as a reading device in an image reader. As shown in FIG. 1, a number of thin oxide films and electrodes are laminated on the surface of the solid-state image pickup device (hereinafter the laminations are referred to as "a surface layer" when applicable). The surface layer causes significant interference in the light incident thereon from particular directions because it has a spectral transmission characteristic as shown in FIG. 2.
In many cases, a picture to be read by the image reader is illuminated by parallel or specular light. Light bearing data or information of the picture (hereinafter referred to as "image data light" when applicable) applied to the solid-state image pickup device has a relatively large number of parallel light components, i.e. is substantially collimated. The aforementioned spectral transmission factor characteristic varies with the thickness and the refractive index of each of the layers forming the surface layer. Accordingly, even when uniform parallel light is applied to the solid-state image pickup device from the light source, if the surface layer is not uniform, for instance, in thickness, the spectral characteristics of light applied to the light receiving section of the image pickup device will be different for different picture elements. That is, the sensitivity of the solid-state image pickup device fluctuates. The variation of the spectral transmission factor characteristics due to this interference increases as the number of parallel light components in the incident light increases.
Accordingly, an object of the invention is to provide an image reader which is very little affected by the above-described interference and accordingly has a smaller sensitivity fluctuation.